* {
    box-sizing: border-box;
}

/*font-family: Arial, sans-serif;*/

header {
    width: 100%;
    height: 150px;
    border: 1px solid black;
    background-color: aquamarine;
    text-align: center;
    margin:0 auto;
    padding-left:calc((100% - 90%)/2) ;/* 水平居中！！！！！ */
    /*overflow: hidden; 清除浮动 */
}

#search-box {
    /* display: block; */
    width: 60%;
    height: 20px;
    background-color: rgb(3, 114, 249);
    float: left;
    margin: 5px auto;
}

#banner {
    width: 90%;
    height: 60px;
    background-color: rgba(179, 4, 4, 0.868);
    float: left;
    margin: 5px auto;
}

#nav {
    width: 90%;
    height: 30px;
    background-color: rgb(3, 114, 249);
    float: left;
    margin: 5px auto;
}

main {
    margin: 0 auto;
    width: 90%;
    border: 1px solid black;
    background-color: aquamarine;
    text-align: center;
    overflow: hidden; /* 清除浮动 */
}

#sidebar {
    width: 20%;
    height: 100%;
    float: left;
}

#login {
    width: 100%;
    height: 260px; 
    background-color: rgb(3, 114, 249);
    margin: 10px;
}


#content {
    width: 80%;
    height: 100%;
    background-color: aquamarine;
    float: left;
}

.box {
    width: 290px;
    height: 260px;
    background-color: rgba(179, 4, 4, 0.868);
    float: left;
    margin: 10px;
}

footer {
    height: 50px;
    background-color: aquamarine;
    border: 1px solid black;
    box-sizing: border-box;
    text-align: center;
    margin: 0 auto;
}